Gas Exchange during Hypoxia and Hypercarbia of Terrestrial Turtles: A Comparison of a Fossorial Species (Gopherus polyphemus) with a Sympatric Nonfossorial Species (Terrapene carolina)

Abstract

Gopherus polyphemus is a terrestrial and fossorial tortoise that spends extended periods in burrows where the CO₂ concentrations may reach 6%-7% and O₂ may fall to 12%. Terrapene carolina is a terrestrial turtle that is sympatric with the gopher tortoise but is not fossorial. The standard metabolic rates (SMR) of the two species are similar when the data are adjusted for differences in body size. The critical oxygen tension ($P_{c}$) of the tortoise (10-11 mmHg) is lower than that of the box turtle (26 mmHg). The $P_{c}$ of both species is unaffected by hypercarbia (6% CO₂). In moderate hypoxia (15% O₂), hypercarbia elicits an increase in $\dot{V}o_{2}$ of 72% in tortoises after 0.75 h of hypercarbia and 206% in box turtles. After 12-18 h of hypercarbia, $\dot{V}o_{2}$ of box turtles returns to normal, while it remains elevated by 22% in gopher tortoises. Both species hyperventilate continuously in hypercarbia, with CO₂ loading occurring in 3-5 h in box turtles and 8-10 h in tortoises; during these period...